Thermionic valve circuits



Allg 28, 1945- P. K. CHATTERJEA ETAL. 2,383,710

THERMIONIC VALVE CIRCUITS Filed March lO, 1944 2 Sheets-Sheet l cz/,QRENT Inventors L r/Rx A Homey Aug- 28, 1945 P. K. CHATTERJEA -r AL 2,383,710

THERMIONIG VALVE CIRCUITS Filed March l0, 1944 2 Sheets-Sheet 2 C/ C2 P+R/+R2 VOL 75465 CURRENT nventons A Homey Patented Aug. 28, 1945 'rmznrnomc VALVE cmcUrrs Prai'ulla Kumar Chatterjea and Charles Thomas Scully, London, England, assignors to Standard Telephones and Cables Limited, London, England, a British company Application March 10, 1944, Serial No. 525,815 In Great Britain May 14, 1943 A (Cl. 179-171) 'I'he present invention relates to thermionic 15 Claims.

valve amplifying circuits, and employs a non-linear resistance element, preferably a thermistor, t obtain an efiicient intervalve coupling which will transmit direct currents.

The lntervalve coupling arrangements in direct current and very low frequency amplifiers present well known difficulties, because a direct current connection between the anode of one valve and the control grid of the next is usually unavoidable, and this means that the anode voltage as well as its variation is transferred to the control grid. In order to prevent this from heavily overbiassing the grid, various cumbersome arrangements involving large counteracting batteries, or separate supplies for each stage, and the like, have been used in the past.

The principal object of this invention is to get over the difficulty by providing a coupling network which transmits direct current and which steps down the mean anode voltage to a much greater degre than the anode voltage variations' due to an applied signal; and the conditions may even be chosen so that it amplifles the voltage variations by stepping them up to the grid while still stepping down the anode voltage.

According to the invention, therefore, there is provided in a multistage thermionic valve amplifier, a network adapted to transmit direct current, for coupling the anode of one valve to the control grid of the following valve, comprising a non-linear resistance element and one or more resistances so selected and disposed that the proportion of the anode voltage variation transferred to the grid is greater than the proportion of the mean anode voltage so transferred.

Alternatively, the invention may comprise in a multistage thermionic valve amplifier, a coupling network adapted to transmit direct currents and to transfer the variations of the anode voltage of one valve substantially unaltered to the control grid of the following valve while applying only a fraction of the mean anode voltage thereto, the network comprising a non-linear resistance element combined with one or more resistances.

According to another aspect, the invention consists in an lntervalve coupling network for a multistage thermionic valve amplifier adapted to transmit direct currents, comprising athermistor having a negative temperature coeiiicient of resistance and one or more resistances so disposed as to transfer amplified anode voltage variations oi one valve to the controlgridy of the next folordinary constant resistance.

lowing valve, while applying a fraction of the mean anode voltage to the said grid.

The invention will be explained with reference to the accompanying drawings in which:

Fig. 1 shows a schematic circuit diagram oian amplifier according to the invention;

Figs. 2 and 4 show characteristic curves t0 explain the operation of Fig. 1; and

Fig. 3 shows a two-stage coupling network according to the invention.

The invention employs in the coupling network a non-linear resistance of semi-conducting material, that is, a resistance in which the relation between the current and the corresponding potential difference is not a straight line as in an This may be a voltage dependent resistance of a carborundum basis or the like, the properties of which are described for instance in the Post Ofiice Electrical Engineers Journal, January 1942 page 180. Preferably, however, the special properties of the temperature dependent resistance elements known as thermistors are employed, since by their use it is possible to obtain amplification in the coupling network.

Thermistors have vbeen in use for some years, and are composed of semi-conducting materials characterized by a temperature coefficient of reslstance which may be either positive or negative and which is moreover many times the corresponding coeiiicient for a pure metal such as copper. Ihis property renders thermistors particularly suitable for a variety of special applications in electric circuits.

Various different materials are available for the resistance element of a thermistor, these various materials having different properties in other respects; as one example, a resistance material havingV a high negative temperature coeilicient of resistance comprises a mixture of manganese oxide and nickel oxide, with orwithout the addition of certain other metallic oxides, the mixture being suitably heat treated.

Thermistors have been employed in two different forms (a) known as a directly heated thermistor and comprising a resistance element of the thermally sensitive resistance material provided with suitable lead-out conductors or terminals, and (b) known as an indirectly heated thermistor comprising the element (a) provided in addition with a heating coil electrically insulated from the element. A directly heated thermistor is primarily intended to be controlled by the current which flows through it and which varies the temperature and also the resistance accordingly. Such a. thermistor will also be affected by the temperature oi its surroundings and may therefore be used for thermostatic control and like purposes with or without direct heating by the current flowing through it. An indirectly heated thermistor is chiefly designed to be heated by a controlling current which flows through the heating coll and which will usually, but not necessarily, be different from the current which flows through the resistance element. but this type of thermistor may also be subjected to either or both of the types of control applicable to e. directly heated thermistor.

More detailed information on the properties of thermistors will be found in an article by G. L. Pearson in the Bell Laboratories Record Dec. 1940, page 106.

In this specification any resistances employed will be assumed to be of the ordinary linear type unless otherwise stated.

Fig. 1 shows a simple two-valve amplifier ernploying an intervalve coupling network according to the invention. The two valves Vi and V2 are supplied with anode voltage from the hightension source connected to the terminal -l-HT through appropriate resistances A. The anode of V1 is connected to the control grid of V2 through a coupling network containing in series a directly heated thermistor T having a resistance element R, together with a resistance R1, and in shunt a resistance R2 connected to the control grid of V2 in series with a small biassing battery B. A signal voltage e is applied to the control grid of V1 at the input terminals i, 2, and the output is taken from the anode of V: through terminals 3, 4 by any appropriate means (not shown). It will be understood that there may be other valve stages before or after Vi and V2 adapted to translate the signal in any way.

The action of the coupling network will be explained with reference to the curves shown in Fig. 2 which relate to the series circuit R, R1, Rz or to parts thereof. The abscissae represent the currents through the circuit to an arbitrary scale, and the ordinates represent the potential differences or voltages also to an arbitrary scale. across the whole or parts of the circuit.

The curve designated R in Fig. 2 is a typical curve showing the relation between the voltage and the current for a thermistor, which is assumed to have a negative temperature coenicient of resistance. This curve has an initial portion with a positive slope for currents up to about 1.2 corresponding to a voltage of 6.9; and afterwards the slope is negative, giving an unstable condition. If a voltage greater than 6.9 ls applied to the thermistor, the current will increase spontaneously until the thermistor is destroyed or until some other factor operates to limit the current.

The straight lines. designated R1 and Ra respectively show the characteristic for the resistances R1 and Rz above. They have a constant positive slope. By adding the ordinates of curves R and R1, the curve R+R1 is obtained and represents the voltage across R and R1 together. Similarly the curve R+R1+R2 represents the voltage v (Fig. l) across the whole combination.

It will be seen that on the curve R+R1 there is a portion between the ordinates C1 and Cz which is approximately nat. 'I'he slope of the curve R-l-.Ri-l-Rs between these ordinates is therefore approximately the same as the slope of the curve Rz. Thus if the voltage v derived from the anode of the valve V1 varies from about 7.6 to 8.2, the voltage across Rz varies from about 1.6 to 2.2. The variation is the same in both cases, namely 0.6 but the mean voltage across Rz, which is applied to the control grid of Vz, is reduced from 7.9 to 1.9.

In other words, the coupling network accordlng to the invention steps down the mean anode voltage without stepping down its variations.

If the mean voltage represented by 1.9 is too large to apply to the control grid, it may be offset by the battery B shown in Fig. l, or by any other convenient biassing arrangement of ordinary type.

The potential difference v is proportional to the signal voltage e. The operating conditions should be chosen according to the manner of variation of the signal: Thus, if the signal varies on either side of a mean value which represents the no signal condition, the resistances R. Ri and R: should be chosen so that the corresponding mean value of v (namely 7.9 in the case of Fig. 2) should produce the current corresponding to the mean of the ordinates C1 and Cz on the curve R+R1+Rz (about 5.55). If, however, the variation is all one way, then the resi condition should correspond to one of the ordinates C1 or C2.

It should be pointed out that if R1 and Re are altered in such a manner that their sum remains constant, the curve R+R1+Rz remains the same. but the curve R-l-Ri is changed. 'I'his facility enables, for example, the resistance Re to be reduced, thereby reducing the mean voltage applied to the grid of V2. The principal effect in the curve R+R1 is to shift the flat .portion to the left, and the range 0f variation of v will at. the same time be reduced.

By using a thermistor having a characteristic curve with a. steeper negative slope, the mean voltage applied to the control grid of Vn may be reduced without reducing the permissible range of variation of v.

It may be further pointed out that it Ri -be reduced and Rz increased by the same amount, the

coupling network may be caused to introduce a voltage amplification. In this case the nat part of the curve R+R1 moves to the right, bringing a portion with a negative slope between the ordinates Ci and Cz. Since the curve R-l-Ri-l-R: is unaltered and since Rz has been increased, the slope of the line Ra will now be greater than the slope of that part of the curve R+Ri+Rz which lieslbetween the ordinates Ci and Cz. In other words, the variation of voltage across R: is now greater than the variation of v.

I1' an indirectly heated thermistor be used instead oi' T, the curve R may be altered by passing a suitable current through the heating coil. The eiIect is mainly to reduce all the ordinates of the curve R, producing a flatter curve Rn, Fig. 2, with a less sharp maximum. The ordinates for large values of current through the element are less.

reduced than for small values. This will enable the operating point to be very easily adjusted-as required. If, for example. the variations of v are small compared with the range defined by the ordinates Ci and Cz, the operating point may be shifted about within this range in order to 11nd the most favourable position.

If it is impracticable to step down the mean anode voltage suillciently in one stage, it can be done in two stages as illustrated in Fig. 3, which shows another network Ta, Ra, R4 and Re connected infront oi the loriginal coupling network' between the anode of Vr and the control grid oi' Va. From what has been said, it will be clear that the mean anode voltage can be stepped down twice, while the variations are substantially unaltered, or may b e ampliiied in each stage. In choosing the values of the second network, account must be taken of the shunting eilect oi' the nrst network on Rs. which must be increased accordingly. It is to be noted that the 4variation of the total resistance R+R1+Rz of the rst network over the operating range is small (only about 20% in the case discussed in connection with Fig. 2) so that as Rs will generally be several times smaller than this total resistance, the performance of the second vnetwork will be little affected by the variation.

The mean anode voltage may clearly be stepped down in more than two stages if desired, by providing the network with any desired number vof additional meshes of the same kind. Any of the thermistors may be of the indirectly heated type, to allow for adjustments of the characteristic as already explained.

While the preferred embodiments of the invention employ one or more thermistors as described, similar results may be obtained by the use of a voltage dependent resistance of the carborundum type described above. It is, however, not possible by this means to obtain an amplifying coupling. The voltage dependent resistance replaces the 'thermistor T and the resistance R1 in Fig. 1.

Fig. 4 shows the characteristic curve (Z)` of the voltage dependent resistance to arbitrary scales. The line Rz is for the resistance R2 as in Fig. 2, and the curve Z+R2 is for the two together obtained by adding the ordinates of the Z and Ra curves. Taking, for example, the nearly straight portion of the curve Z+Rz between the ordinates corresponding to currents of 3 and 5, it will be seen that the corresponding anode voltage variation is from about 4.5 5.8, that is, a total variation of 1.3, the mean voltage beingabout 5.15. The corresponding variation of voltage across R2 is from 0.9 to 1.5, a total of 0.6, the mean voltage being 1.2. Thus the voltagevariation applied to the control grid is a little less than half the anode voltage variation, but the mean voltage applied has been stepped down to less than a quarter.

AThe advantage will be increased by making Rz smaller, but this will atthe same time reduce the voltage variation available at the control grid.l

It will be evident that any or all of the series arms of a network like Fig. 3 with two or more meshes may be replaced by voltage dependent resistances.

The counteracting bias for the valve V2 may be provided without the use of the battery B, if preferred. In this case the resistance Rz is connected directly to earth. The two cathodes are then connected together and to earth through the resistance element of an additional thermistor (not shown), having a negative temperature coeiiicient of resistance. This thermistor should preferably be of the indirectly heated type, and an appropriate current should be passed through the heating coil from a local circuit to produce a characteristic curve like Rn (Fig. 2), having a fairly fiat maximum at M. The thermistor should be selected so that the combined cathode currents which flow through it correspond to the abscissa of the point M, and the corresponding ordinate should at the same time correspond to the desired counteracting voltage. By this means the value of this voltage will be substantially independent of the variations inthe cathode currents caused by the signal, and the necessity for the battery B is avoided. This arrangement may be adopted when the coupling network has any number of meshes.

It will be understood that the characteristic feature of this invention is a coupling network which transmits direct current, but which applies to the control grid a proportion of the anode voltage variation larger than the proportion of the mean anode voltage so applied. In the claims the word proportion is to be understood to include proportion values greater than l.

Whatis claimed is:

1. In a multi-stage thermlonic valve amplifier. means for coupling the anode of onevalve to the control grid of the following valve, said means including a. network adapted to transmit direct current and comprising a non-linear resistance element and at least one ordinary resistance s0 selected and disposed that the proportion of the anode voltage variation transferred. to the grid is greater than the proportion of the mean anode voltage so transferred.

2. In a multi-stage thermlonic valve amplifier, means for coupling the anode of one valve to the control grid of the following valve, said means including a coupling network comprising a nonlinear resistance element and at least one ordi nary resistance adapted to transmit direct currents and to transfer the variations of the anode voltage of one valve substantially unaltered to the control grid of the following valve while applying only a fraction of the vmean anode voltage thereto.

3. An amplifier according to claim 1,y in which the non-linear resistance element is a thermistor and has a negative temperature coeiiicient of resistance.

4: An amplifier according to claim 2, in which the non-linear resistance element is a thermistor and has a negative temperature coefficient of resistance.

5. An amplier according to claim l, in which the network comprises a thermistor having a negative temperature coefiicient of resistance and at least one ordinary resistance so disposed as to transfer 'amplified anode voltage variations of one valve to the control grid of the next following valve, while applying a fraction of the mean anode voltage to the said grid.

6. An amplifier according to claim 1, inwhich the non-linear resistance element is a voltage dependent resistance of the carborundum type.

7. An amplifier according to claim' l, in which the non-linear resistance element is a voltage dependent resistance of the carborundum type, and connected in series between the anode and the control grid, while at least one of the ordinary resistances is connected in shunt between the control grid and cathoderof the following valve.

8. An amplifier according to claim 1, in which the network comprises a thermistor having a negative temperature coefficient of resistance connected in series with a first ordinary resistance between the anode and thecontrol grid, and a second ordinary resistance -connected in shunt between the controlgrid and cathode of the'following valve.

9. In a multi-stage thermlonic valve amplifier, means for coupling the anode of one valve to the control grid ofthe following valve, said means including a network adapted to transmit direct current and comprising a non-linear resistance element and at least one ordinary resistance so selected and disposed that the proportion of the anode voltage variation transferred to the grid is greater than the proportion of the mean anode voltage so transferred, a non-linear resistance element comprising a thermistor having a negative temperature coetilcient of resistance, the resistance element of the thermistor being connected in series with a rst resistance between the anode and the control grid, a second resistance being connected in shunt between the control grid and cathode of the following valve.

l0. In a multi-stage thermionic valve amplifier, means for coupling the anode of one valve to the control grid of the following valve, said means including a coupling network adapted to transmit direct current and to transfer the variations of the anode voltage of one valve substantially unaltered to the control grid of the following valve while applying only a fraction of the mean anode voltage thereto, said network comprising a nonlinear resistance including a thermistor and at least one ordinary resistance, the operating point of the thermistor being set between ordinates which bound a substantially flat portion of the characteristic curve of the voltage dependent on the surn of said resistances.

l1. An amplifier according to claim 1, in which the network comprises a thermistor having a resistance element with a negative temperature coefficient of resistance, connected in series with a iirst ordinary resistance between said anode and said control grid, and a second ordinary resistance connected in shunt between the control grid and the cathode of said following yalve.

12. An ampliiler according to claim l, in which the network comprises a thermistor having a resistance with a negative temperature coefficient of resistance, connected in series with a rst ordinary resistance between said anode and said control grid, and a second ordinary resistance connected in shunt between the control grid and cathode of said following valve, the operating point of the thermistor being set between ordinates which bound a substantially nat portion of the charac* teristic curve of the voltage dependent on the sum of said three resistances.

13. An amplifier according to claim 1, in which the network comprises a thermistor having a resistance with a negative temperature coemcient oi' resistance connected in series with a first ordinary resistance between said anode and said control grid, and a second ordinary resistance connected in shunt between the control grid and cathode of said following valves, the operating point of the thermistor being set on an ordinate of the characteristic of the voltage dependences which cuts the characteristic curve of the voltage dependent on the sum of the two first-mentioned resistances at a point of negative slope and cuts the characteristic curve of the voltage dependent on the sum of all three resistances at a point of positive slope.

14. In an amplifier according to claim l, a second network connected in front of the original coupling network between said anode and said control grid.

l5. In an amplifier according to claim l, a second network connected in front ot the original coupling network between said anode and said control grid. comprising at least one thermistor of the indirectly heated type.

PRAFULLA KUMAR CHA'I'I'ERJEA. CHARLES THOMAS SCULLY. 

